1. Field of the Invention
This invention relates to the measurement of blood flow in a subject, more particularly to a method and apparatus for measuring pulmonary blood flow by pulmonary exchange of oxygen and an inert gas with the blood utilizing a divided respiratory system. The invention is especially suitable for monitoring pulmonary blood flow/cardiac output of a patient under general anesthetic and accordingly it will be convenient to described the invention in connection with this application. However, it is to be understood that the method and apparatus described herein may be used for determining the pulmonary blood flow or cardiac output of a subject in a conscious state.
2. Description of the Related Art
The equation that links the cardiac output of a subject to more directly measured parameters is as follows:
xe2x80x83{dot over (U)}gas={dot over (Q)}cxcex(FAgasxe2x88x92F{overscore (V)}gas)
where FAgas refers to the concentration of inert soluble gas in the alveolar gas mixture of the lungs expressed as fraction of its partial pressure to the barometric pressure (Bp),
F{overscore (V)}gas refers to the fraction of the inert soluble gas in the mixed venous blood expressed as a fraction of its partial pressure to the total pressure,
xcex is the Ostwald solubility coefficient of the inert soluble gas in blood,
{dot over (Q)}c is the cardiac output that passes through the pulmonary capillaries in the walls of gas containing alveoli, and
{dot over (U)}gas is the uptake into the blood from the alveoli measured in units of volume at body temperature and barometric pressure per unit time.
This equation holds true for inert gases only. In this regard an inert gas dissolves in blood proportionally to its partial pressure i.e. it obeys Henry""s Law. By contrast a reactive gas does not obey Henry""s Law by reason of its reacting chemically with blood constituents. Oxygen and carbon dioxide are examples of reactive gases.
The term cardiac output as used herein refers to the amount of blood per unit time which passes through the pulmonary capillaries in the walls of the alveoli of the lungs. If hemoglobin O2 saturation of the subject is 100% then the whole cardiac output will be equivalent to the pulmonary blood flow, i.e. the amount of oxygenated blood passing through the pulmonary capillaries in the walls of the alveoli of the lungs. If this saturation is less than 100% the whole cardiac output includes shunt blood in addition to pulmonary blood flow. Shunt blood does not transport O2 from the lungs to the tissue and may therefore be ignored. The % shunt may be estimated from pulse oximetry.
Most methods in use today or described in the literature refer to or depend on the above equation, but F{overscore (V)}gas cannot be measured accurately without obtaining a sample of mixed venous blood, which would sacrifice the advantage of non-invasiveness of large blood vessels with catheters, as is necessary with the most widely used method of measuring cardiac output presently in use, namely the thermodilution method.
Most gas exchange methods for measuring the cardiac output which have been attempted suffer from the problem of xe2x80x9crecirculationxe2x80x9d which limits them to only intermittent determinations of {dot over (Q)}c separated by relatively long intervals to wash out gas introduced by the previous determination. This restriction of frequency of taking readings of {dot over (Q)}c is necessary to ensure that F{overscore (V)}gas has returned to a value close to zero before another determination is performed. The same constraint also applies to methods using reactive gases. The term xe2x80x9crecirculationxe2x80x9d refers to the return back to the lungs in the mixed venous blood of gas that has previously been taken away from the lungs in the arterial blood.
It is an object of the present invention to overcome or at least alleviate one or more of the abovementioned difficulties of the prior art, or at least to provide the public with a useful choice.
Accordingly, in a first aspect the present invention provides a method for measuring the pulmonary blood flow in a subject including:
isolating two or more divisions of the respiratory system, said divisions comprising the complete gas exchanging part of said respiratory system,
ventilating each said division with a separate gas mixture, at least one of said gas mixtures including an inert soluble gas,
determining uptake of inert soluble gas in at least two of said divisions,
determining relative pulmonary blood flow of said divisions,
determining end tidal concentration of inert soluble gas in at least two of said divisions, and
calculating pulmonary blood flow from determined values of uptake and end tidal concentration of inert soluble gas, and relative pulmonary blood flow.